Title

U.S. Department of the Interior

U.S. Geological Survey

Probable Production Induced Subsidence, Fault Reactivation, and Wetland Loss in the

Gulf Coast Region

R. Morton, N. Buster, D. Krohn, and R. Peterson

U.S. Geological Survey Center for Coastal and Watershed Studies

St. Petersburg, FL 33701

http://coastal.er.usgs.gov/gc-subsidence/

SCIENTIFIC OBJECTIVES

• Investigate timing of subsidence and faulting

• Compare geological and historical rates of subsidence

• Evaluate geological and engineering factors that may influence induced subsidence and faulting

• Examine mechanisms of subsidence and faulting

• Evaluate methods of regional subsidence detection and monitoring

• Develop predictive capabilities (subsidence susceptibility)

ENVIRONMENTAL APPLICATIONS

• Evaluate potential contributions to historical wetland losses

• Incorporate into resource management and coastal restoration plans

• Evaluate methods of subsidence mitigation

EVIDENCE OF INDUCED SUBSIDENCE AND FAULT REACTIVATION

• Temporal and spatial correlation of surficial changes and hydrocarbon production

• Large or rapid subsurface pressure decline (regional depressurization)

• Consistent orientation and displacement of active surface and subsurface faults

• Historical subsidence rates significantly greater than geological subsidence rates

• Preservation of marsh sediments beneath open water (historical wetland loss)

COCODRIE TIDE GAUGE

Annual production Lirette

NGS Data 1965 - 1982

NGS Data 1966 - 1993

Average geological and historical rates of subsidence for the Terrebonne delta plain region near Madison Bay.

DELTA PLAIN WETLAND LOSS

• Prior studies attributed regional wetland loss to delta plain compaction, canal construction, and biogeochemical processes

• Subsidence associated with natural compaction should be slow and decrease with geologic time

• Some delta plain subsidence rates accelerated recently and are greater than geologic subsidence rates

CONCLUSIONS

• Marsh sediments are preserved where accommodation space is created by induced subsidence

• Prior explanations of regional wetland loss fail to explain the rapid increase and decrease in rates of loss (1950s-1970s)

• Close temperal and spatial correlations among regional wetland loss, highest historical subsidence rates, maximum rates of fluid extraction and pore-pressure reduction, and locations of potentially reactivated faults

• Decrease in wetland loss possibly related to decreased subsidence associated with decrease oil & gas production